Method and device for the non-destructive testing of objects using ultrasonics

ABSTRACT

The invention concerns a method and device for nondestructive testing. Ultrasonic pulses are fed into the object under test. Ultrasonic waves emerging from the object are detected by an ultrasonic test head, converted into electrical signal and amplified. The signals are then sampled to produce measurement values, which are digitized and stored. From the digitized sampled values of the signals a function is obtained by interpolation, by means of which the peak values of the ultrasonic waves and/or the time of propagation of the wave at the peak is determined.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method and a device for non-destructivetesting of objects using ultrasonics, whereby ultrasonic pulses areintroduced by an ultrasonic testing head into the respective object, andwhereby ultrasonic waves are emitted by the object in response to theultrasonic pulses introduced and are then picked up by an ultrasonictesting head and converted into electrical signals of a signal sequence.

2. Prior Art

In medical technology, an ultrasonic examination method for diagnosticpurposes is known in which ultrasonic vibrations are introduced into abody to be examined that then generates ultrasonic echoes which areconverted by an ultrasonic converter into electrical signals. Theradiation direction of the ultrasonic vibrations is gradually changed inidentical angular steps using mechanical or electronic means. Anexamination area in the body is limited by the two outer radiationdirections. The ultrasonic echoes reflected along the paths preset bythe radiation directions are converted into a sequence of measurementvalues that represent the reflection properties of the examination areaat various measurement points that are associated with picture points ofan ultrasonic image displayed on a monitor. For each picture point ofthe ultrasonic image, a picture value is calculated by interpolation ofthe measured values of those measurement points adjacent to the picturepoint as a result of the geometrical association between the examinedarea and the ultrasonic image (DE 36 40 882 A1).

In medical technology, a method is also known for delay determination ofultrasonic pulses passing through an examined area and converted by anultrasonic converter into an electrical measurement signal. Afteremission of the ultrasonic pulse, the time that passes until themeasurement signal has reached a preset threshold is measured. Themeasurement signal is amplified and fed to an analog/digital converterworking with a predetermined sampling rate. The sample values of theanalog/digital converter are stored. An excerpt from the measurementsignal formed by a sequence of sample values is compared with anidentical-length excerpt of a reference signal stored as a sequence ofreference values. The excerpt of the reference signal and if necessarythat of the measurement signal are displaced one node at a time untilthe displaced sections match up best when they are again compared withone another. The delay is determined by a digital processor, which canalso have controlling function, by correction of the period thusascertained. The sequence of reference values, which is if possibleprecisely proportional to the sample values of the excerpt, is foundwith a function which is a yardstick for the divergence of themeasurement signal excerpt scaled with one factor from a correspondinglylong excerpt of the reference signal (DE 32 42 284 A1).

Also known is an ultrasonic imaging device for medical diagnostictechnology, having an ultrasonic converter that introduces an ultrasonicbeam into a body in order to achieve sector sampling. The ultrasonicconverter converts ultrasonic echo waves into echo signals that aresampled in intervals that are shorter than the division of the pixels orpicture elements. The measurement values sampled along each samplingline are stored as pixel data. The data disposed at those points of twoadjacent sampling lines that are identically spaced from a beam emissionpoint are used to obtain interpolation data (DE 36 32 813 A1).

In a known method for determining the amplitude and the amplitudeposition of the maximum of a correlation signal, a correlator issupplied with a digitalized picture signal and a digitalized referenceimage. The correlator generates the correlation signal, which is passedto a memory with maximum detector that calculates the amplitude and theamplitude position. The calculated values are then interpolated in orderto determine the real maximum of more closely approximated values (DE 3812 195 A1).

Finally, it is known in measurement signal processing withhigh-resolution analog/digital converters to filter the analog signalswith low limit frequency and to conduct the analog/digital conversionwith high resolution (periodical: Technisches Messen tm, 52nd year,issue 11, 1985, pages 404-410).

It is furthermore known to interpolate discrete values of a function bypolynomials (Book: A. Duschek: "Vorlesungen uber hohere Mathematik", 4thedition 1965, pages 297-301, Vol. 1).

Important characteristic quantities for an ultrasonic instrument fornon-destructive materials testing in automated testing facilities arethe speed and the accuracy with which crack fault signals and wallthickness signals received from fault echoes generated using theultrasonic pulse echo method or from a rear-wall echo sequence can bepicked up.

Accordingly, the quality of a ultrasonic instrument hinges on theamplitude and delay resolution achieved with a pulse sequence frequencyas high as possible. By amplitude resolution we understand the accuracywith which the amplitude extreme is ascertained (positive maximum ornegative minimum), or the positive or negative maximum pulse peak. Thedelay is the time passing between the entry of the ultrasonic pulsesinto the object and the reception of the ultrasonic waves leaving theobject.

In an analog ultrasonic instrument, various time intervals (windows) canbe selected for evaluation of the signals received from a testing heador ultrasonic sensor after amplification. For each window, a peak valuememory, using which the signal extreme within the window is recorded inanalog form, is necessary for amplitude determination. In conventionalsystems, the signal thus obtained is then digitalized for furtherprocessing in a computer.

For measurement of the delay, an additional module is necessary. Withthe signal peak or the flank, in the case that the threshold within awindow is exceeded, a digital counter is started which is stopped withthe result within a second window. To achieve the required precision,high-frequency and hence expensive digital counters are necessary here.To improve the resolution, an additional analog time measurement isoften integrated using a sawtooth signal. The sawtooth amplitude valueachieved at the stop event is also digitalized and converted by acalibration unit into a delay.

In a digital ultrasonic instrument, the complete signal sequencereceived from an ultrasonic testing head or ultrasonic sensor isdigitalized immediately after amplification. The amplitude and the delayare then determined from the digital data.

The accuracy with which high-frequency pulse-like signals can bedetermined by digital measurement value pickup is limited by theperformance data of the analog/digital converter (ADC) used. Theachievable accuracy when determining the amplitude of the maximum pulsepeak is determined predominantly by the sampling rate (samplingfrequency) of the ADC, beside the digitalization definition (bitnumber). The time position of the pulse maximum--precise knowledge ofwhich is required for determining the delays--is determined solely bythe available sampling rate.

The lower the ratio of sampling frequency to signal frequency, thepoorer the resolution of the amplitude and delay determination. Thismeans that to determine short wide-band and hence high-frequency pulses,very high sampling rates are necessary.

For achieving the required accuracy, technology normally applies methods(interleaved or random sampling) in which the same event must occurseveral times consecutively and then be digitalized at different times.To achieve the required accuracy in the case of events that occur onceonly, however, demands are placed on the ADC to be used (highdigitalization and high sampling rate) and on the following-on memorymodules (high speed and great depth) that are at present not feasiblefor price reasons on account of the very high expenditure involved.

If a commercially available analog-digital converter and standard memorymodules are used, the sampling density of the signals is not sufficientfor precise determination of the maximum signal amplitude and itstiming.

SUMMARY OF THE INVENTION

This is where the invention comes in: the problem underlying theinvention is to develop a method and a device for non-destructivetesting of objects using ultrasonics, whereby with a preset samplingrate for the signals generated by an ultrasonic receiver the amplitudemaximum and/or the delay of the amplitude maximum can be determined withhigh resolution.

The problem is solved for the method according to the invention in thatmeasurement values are obtained from the signals of the signal sequenceby time-equidistant sampling and then are digitalized and stored, andthat a function represented by the discrete measurement values storedand digitalized is replaced by a Grade n polynomial according to thefollowing equation: ##EQU1##

where t_(i) is the sampling time, for i=0,1 . . . N, and a_(n) thecoefficients of the polynomial, and in that the amplitude extreme of thereceived ultrasonic waves and/or the delay of the ultrasonic waves withthe amplitude extreme is determined using the polynomial. With the aidof the interpolation method, it is then possible to achieve a highresolution when determining the amplitude and the delay, even ifcommercially available modules are used for the digitalizer and thememories. In addition, the extent of the hardware is reduced comparedwith typical analog-operating ultrasonic testing systems. The amplitudeand delay measurements can still be performed with the same hardware.The amplitude extreme is compared during further processing inparticular with at least one preset threshold value that ischaracteristic for a fault.

By interpolation with the polynomial, the result can be obtained withrelatively little calculation work.

It is expedient if the delay of the ultrasonic waves is determined withthe amplitude extreme from the first derivation of the Grade Npolynomial based on the time using the following equation: ##EQU2##

and that the amplitude extreme is determined by incorporation of thedelay value t=t₀ into the equation ##EQU3##

of the polynomial.

The signals emitted by the ultrasonic testing head are preferablysampled with a sampling rate between 60 and 120 MHz and digitalized witha resolution of eight to twelve bits. These sampling rates can beachieved with commercially available sample and hold circuits. Forresolutions up to 8 to 12 bits, commercially available analog/digitalconverters can be used.

In a favorable embodiment, the sample values are filtered to reducefault effects.

A device for non-destructive testing of objects using ultrasonics, bywhich on the one hand ultrasonic pulses are introduced by an ultrasonictesting head into the respective object and on the other hand ultrasonicwaves being emitted by the object are picked up by an ultrasonic testinghead and converted into electrical signals of a signal sequence, wherethe device has an amplifier, an analog/digital converter, a write/readmemory, a digital processor and a bus, consists in accordance with theinvention in that a sample and hold circuit is connected in front of theanalog/digital converter and operates with a sampling rate in the rangefrom 60 to 160 MHz to generate from the electrical signals measurementvalues that are digitized by the analog/digital converter with aresolution of twelve bits and then stored in the write/read memory, andin that the discrete, stored and digitized measurement values form afunction which is replaced by a Grade N polynomial according to thefollowing equation: ##EQU4##

where t_(i) is the sampling times for i=0.1 . . . N, and a_(n) thecoefficients of the polynomial, and in that the amplitude extreme of theultrasonic waves and/or the delay of said ultrasonic waves with theamplitude extreme is determined using said polynomial. This arrayoperates with a commercially available sample and hold circuit and acommercially available analog/digital converter. The higher resolutionsof the amplitude and the delay can therefore be achieved in aneconomical manner.

Preferably, at least two devices each having an ultrasonic testing headfor transmission and reception, an amplifier, one sample and holdcircuit each, one write/read memory each and a processor are combined inone design unit. Since the hardware expenditure for evaluation of thesignals generated by an ultrasonic receiver is relatively low, a numberof such arrays can be provided parallel to one another for increasingthe testing speed.

Since the complete signal curve is available in digital form, all theadvantages of digital signal processing such as digital filtering orcorrelation can also be used for improvement of the accuracy.

Further details, advantages and features of the invention are clear notonly from the claims and from the features they describe, singly and/orin combination, but also from the following description of a preferredembodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a device for non-destructive testing of objects withdigital processing of received ultrasonic signals in a block diagram,

FIG. 2 shows a block diagram of a conventional, analog-operating devicefor non-destructive testing of objects,

FIG. 3 shows a diagram of the theoretically possible amplituderesolution as a function of the sampling rate and the digitalization ofan analog/digital converter,

FIG. 4 shows a digitalized amplitude signal, received as a crack faultecho, of an ultrasonic receiver as a function of the time,

FIG. 5 shows a digitalized amplitude signal, received as a wallthickness echo sequence, as a function of the time,

FIG. 6 shows a digitalized amplitude signal received as a crack faultecho, and an interpolation curve for the amplitude signal as a functionof the time,

FIG. 7 shows a digitalized amplitude signal received as a wall thicknessecho sequence, and an interpolation curve for the amplitude signal as afunction of the time,

FIG. 8 shows the amplitude resolution of a certain interpolation methodas a function of the sampling rate and of the resolution of theanalog/digital converter, and

FIG. 9 shows the delay resolution of a certain interpolation method as afunction of the sampling rate and of the resolution of theanalog/digital converter.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

A device for non-destructive testing of objects using ultrasonics isprovided with an ultrasonic testing head (10) for transmittingpulse-like ultrasonic waves and an ultrasonic testing head (12) forreceiving ultrasonic waves. The transmitter and the receiver forultrasonic waves are frequently combined in one testing head. Theelectro-acoustic ultrasonic testing head (10) can be connected using achangeover switch (14) to a transmitter (16) that generates pulses orpulse groups. The transmitter (16) can also be connected to theultrasonic testing head (12) using the changeover switch (14). Anamplifier (18) is connected to the ultrasonic testing head (12), towhich is connected a sample and hold circuit (20). The sample and holdcircuits (20), an analog/digital converter (24), a write/read memory(26) and a processor (28) are connected using a data bus (22).

FIG. 2 shows an analog-operating device for non-destructive testing ofobjects. The device according to FIG. 2 has, like the device shown inFIG. 1, the receiver (16), changeover switch (14), the two ultrasonictesting heads (10), (12) in the form of electro-acoustic converters, andthe amplifier (18). Using a window control and evaluation unit (30),various time intervals (windows) the selectable for evaluation of thesignals received from the ultrasonic testing head (12) afteramplification. For each window, a peak value memory is necessary foramplitude determination. FIG. 2 shows three peak value memories (32),(34) and (36). With the respective peak value memory, the signal maximumwithin the window is picked up in analog form. The signal thus obtainedcan then be digitalized in a computer and further processed. For delaymeasurement, a module (38) is required in addition. When the thresholdis exceeded within a window, a digital counter is started with thesignal peak or flank, and stopped with the event within a second window.To achieve the required accuracy, high-frequency and hence expensivecounters are necessary. To improve the resolution, an additional analogtime measurement is often integrated using a sawtooth signal. Theamplitude value of the saw-tooth achieved at the stop event is alsodigitalized and converted into a delay by a calibration unit.

With the ultrasonic testing instrument shown in FIG. 1, the signalsequence received from the ultrasonic testing head (12) is, afteramplification, sampled, digitalized and stored. The amplitude maximumand the delay of the ultrasonic pulses until the amplitude maximum isreceived are determined from the stored data. FIG. 3 shows athree-dimensional diagram in which the amplitude error is shown in % asa function of the sampling rate in MHz and of the digitalizationresolution in bits.

FIG. 3 shows that the achievable accuracy when determining the amplitudeof the maximum pulse peak is determined predominantly by the samplingfrequency of the sample and hold circuit or of the analog/digitalconverter, beside the digitalization resolution. The accuracy of thetime position of the pulse maximum--precise knowledge of which isrequired for determining the delays--is determined solely by thesampling rate. The lower the ratio of sampling frequency to signalfrequency, the poorer the resolution of the amplitudes and delays.

In the array shown in FIG. 1, commercially available components, e.g. acommercially available sample and hold circuit (20), a commerciallyavailable analog/digital converter (24) and a commercially availablewrite/read memory (26) of RAM type are used. The analog/digitalconverter (24) has a resolution of 8-12 bits. The sampling rate(sampling frequency) of the sample and hold circuit and of theanalog/digital converter is in the range from 60 to 160 MHz.

FIG. 4 shows a typical echo signal (40) generated by a crack fault withits sample values, connected by straight lines in FIG. 4, as a functionof the time t. FIG. 5 shows the sample values for a typical wall echosequence (42) that are interconnected by straight lines, as a functionof the time t. The equidistant sampling times shown in FIGS. 4 and 5 areachieved using commercially available sampling rates. It is clear thatthe sampling density in accordance with FIGS. 4 and 5 is not sufficientfor precise determination of the maximum signal amplitude and of thetime of occurrence of the maximum signal amplitude.

In order to achieve, using the commercially available modules forgeneration of the sampling rate and digitalization, a higher resolutionwith regard both to the maximum signal amplitude and to the timing ofthe signal amplitude, the function stored and given by the digitalsample values is interpolated.

Interpolation is based on the knowledge that the electrical measurementsignal with the time function f(t) at the connections of an ultrasonictesting head, e.g. of ultrasonic testing head (12), has the followingproperties:

    |f(t)|<ε.sub.t, for t<T.sub.s und t>T.sub.e (1)

with ε_(t) as a barrier as small as required and T_(s) and T_(e) as thestart and end times of the ultrasonic signal sequence.

Transformed into the frequency range, the following applies:

    |F(ω)|<ε.sub.ω, for ω<ω.sub.s und ω>ω.sub.e           (2)

with ε.sub.ω as a barrier as small as required and ω_(s) and ω_(w) asthe lower and upper limit frequency.

Here, F(ω) is the Fourier transformation based on the followingequation: ##EQU5##

These are time-limited and band-limited signals.

The ultrasonic testing heads are generally vibration-capable, resonantstructures of which the output voltages can be approximately describedas amplitude-modulated sinusoidal vibrations and hence have theproperties stated in the equations (1-3).

The time function f(t) is, in the case of the device shown in FIG. 1,present in the form of time-equidistant, discrete sample values, i.e.the following applies:

    f(fΔt), for i=0, 1, . . . M                          (4)

The sampling intervals are sufficiently small, so that the followingapplies: ##EQU6##

When the condition set forth in (5) is met, the function values betweenthe sampling times nat and (n+1) At can be ascertained by interpolation.After determining the parameters of the interpolation, the extremevalues of the amplitudes and the delays until occurrence of the extremevalues are stated analytically. A grade n polynomial is selected as theinterpolation function.

Other functional approaches are possible, however these generally leadto a non-linear system of function equations that can only be solvediteratively--and hence in a calculation-intensive form. Using thepolynomial: ##EQU7##

the sum of the mean fault square is obtained. ##EQU8##

From the requirement that the sum of the divergence squares be aminimum, the following results: ##EQU9##

With the abbreviations ##EQU10##

the solution of the equation system in (8) can be provided:

    a.sub.k =b.sub.kn.sup.-1 c.sub.n ;n=0, 1, 2, . . . N       (10)

with b⁻¹ kn to be understood as the elements of the inverse matrixassociated with the matrix elements b_(kn).

If N=M is selected, a strict interpolation results, i.e. F_(Q) =0. FromM>N, a compensating curve is obtained and the interpolation is redundantwith M<N.

The amplitude maximum is determined as follows:

First the time of the amplitude extreme is calculated from the followingequation: ##EQU11##

For N=2, the following results with the coefficients a₁ and a₂ :##EQU12##

For N=3, it follows with the coefficients a₁, a₂ and a₃ : ##EQU13##

For N=4, three solutions are obtained with the Cardan solution formula.If N>4 is selected, the solution can generally only be determinediteratively (for example Bjork --Anderson method).

The amplitude maximum is determined by incorporating the appropriatetime value into the polynomial formulation given above in (6). Athreshold value determination takes place in analogous form.

An additional difficulty in the determination of amplitudes and delaysis encountered when the measured function values f_(M) (ti) are subjectto a statistical uncertainty:

    f.sub.M (t.sub.i)=f(t.sub.i)+r(t.sub.i)                    (14)

In r(ti), all the error sources are summarized:

--discretization noise (AD converter)

--"ultrasonic grass" (structure noise)

--electronic noise (signal processing electronics)

--interference signals (crosstalk of surrounding electronics)

To reduce these error effects, the function f_(M) (ti) is subjected tofiltering. In the frequency range, the following is obtained:

    F.sub.fl (ω)=F.sub.M (ω)                       (15)

and in the time range, with determination of the output signals usingthe convolution integral: ##EQU14##

If filtering is only necessary within a short time, the convolutionintegral can be reduced approximately to a very limited integration andcalculation interval. This is numerically simpler and faster than thecalculation of the equations (15-16).

For the filter function, an ideal low-pass filter with the followingtransmission function is conceivable in the simplest case: ##EQU15##

A considerably better possibility is represented by a matched filter (orcorrelation filter):

    h(t-τ)=f.sub.sample (τ-t-t.sub.0)                  (18)

As f_(sample), the first A-sample of a measurement series is used, forexample. The time of the signal maximum is designated as t_(o), so thatthe convolution result does not appear with a time lag.

In FIG. 6, digitalized measurement values for a typical crack fault echoare marked in a diagram as crosses (43) and connected to one another bystraight lines (44). This shows the course of the crack fault echosignal as a function of the time t, shown in the abscissa direction.Furthermore, interpolation curves (46), (47) and (48) are shown as afunction of the time t in FIG. 6. The interpolation curves (46) to (48)were determined according to the polynomial formulation described above.

Furthermore, FIG. 6 shows the amplitude extreme (49) of theinterpolation curves (46) to (48). A high resolution can be achievedusing the amplitude extreme determined from the interpolation curves, asFIG. 6 shows.

FIG. 7 shows digitalized measurement values for a typical wall thicknessecho sequence, marked by crosses (50) in the diagram and joined up bystraight lines (51).

This shows the course of the wall thickness echo sequence as a functionof the time t, shown in the abscissa direction.

FIG. 7 also shows interpolation curves (52), (53), (54) as a function ofthe time t. The interpolation curves (52) to (54) were determined on thebasis of the above interpolation formulation using the digitalizedmeasurement values plotted in FIG. 7. The amplitude extreme (56)obtained on the basis of the interpolation curves (52) to (54) is alsoplotted in FIG. 7. This FIG. shows that with the interpolation curvesthe amplitude extreme can be determined with higher resolution than withthe digital sample values alone.

The theoretically possible resolution by means of interpolation for thesignal amplitude is shown in FIG. 8 in the three-dimensional diagram in% as a function of the sampling rate (sampling frequency) in MHz and ofthe resolution of the analog/digital converter (24) in bits.

FIG. 9 shows the resolution theoretically possible with interpolationfor the delay of the ultrasonic signals in steel in μm as a function ofthe sampling rate (sampling frequency)in MHz and of the resolution ofthe analog/digital converter (24) in bits.

Several devices shown in FIG. 1 are best combined in one design unit,using which objects are non-destructively tested in several tracks. Thisallows an increase in the testing speed.

What is claimed is:
 1. A method for use in non-destructive testing ofobjects using ultrasonics, comprising the steps of:introducingultrasonic pulses by an ultrasonic testing head into an object;receiving by an ultrasonic testing head ultrasonic waves emitted by theobject in response to the ultrasonic pulses and converting theultrasonic waves into electrical signals having a signal sequence;obtaining measurement values from said signals of said signal responsesby time-equidistant sampling; digitizing and storing said measurementvalues; replacing a function represented by the digitized and storedmeasurement values by a Grade N polynomial according to the followingequation: ##EQU16## wherein t_(i) is the sampling time, for i=0, 1 . . .N, and a_(n) the coefficients of the polynomial; and using saidpolynomial, determining with high resolution an amplitude extreme of thereceived ultrasonic waves and/or a delay of said ultrasonic wave with anamplitude extreme.
 2. A method according to claim 1, wherein said delayof said ultrasonic wave is determined with the amplitude extreme fromthe first derivation of said Grade N polynomial based on time using thefollowing equation: ##EQU17## and said amplitude extreme is determinedby incorporation of the delay value into the equation ##EQU18## of saidpolynomial.
 3. A method according to claim 1 or claim 2wherein theelectrical signals are sampled with a sampling rate between 60 and 120MHz and are digitized with a resolution of eight to twelve bits.
 4. Amethod according to claim 3, further comprising:filtering saidmeasurement values.
 5. A method according to claim 4, wherein saidfiltering has a low-pass characteristic.
 6. A method according to claim4, wherein said filtering has a matched-filter characteristic.
 7. Adevice for use in non-destructive testing of objects using ultrasonics,comprising:an ultrasonic testing head for introducing ultrasonic pulsesinto an object and for receiving ultrasonic waves emitted by the objectin response to the ultrasonic pulses and converting the ultrasonic wavesinto electrical signals having a signal sequence; an amplifier forreceiving said electrical signals; an analog/digital converter; a sampleand hold circuit connected between the amplifier and the analog/digitalconverter and operating with a sampling rate in the range of 60 to 120MH_(z) to generate from said electrical signals measurements which aredigitized by said analog/digital converter; a write/read memory joinedto said converter for storing said digitized measurement values; and adigital processor joined to the write/read memory for processing thestored digitized measurement values to replace said values by a Grade Npolynomial according to the following equation: ##EQU19## wherein t_(i)is the sampling time, for i=0, 1 . . . N, and a_(n) the coefficients ofthe polynomial; and for using said polynomial to determine an amplitudeextreme of the received ultrasonic waves and/or a delay of saidultrasonic wave with an amplitude extreme.